I have a problem building an aggregate function. Here's my problem: I have a table like this id action day isSum difference 1 ping 2012-01-01 1 500 (this is the sum of the differences from last year) 2 ping 2012-01-01 0 -2 3 ping 2012-01-02 0 1 4 ping 2012-01-03 0 -4 5 ping 2012-01-04 0 -2 6 ping 2012-01-05 0 3 7 ping 2012-01-06 0 2 8 ping 2012-01-01 1 0 (this is the sum of the differences from last year, now for pong) 9 pong 2012-01-01 0 -5 10 pong 2012-01-02 0 2 11 pong 2012-01-03 0 -2 12 pong 2012-01-04 0 -8 13 pong 2012-01-05 0 3 14 pong 2012-01-06 0 4 I now need to select the action, day and the summarized difference since 01-01 for every day, so that my result looks like this action day total ping 2012-01-01 498 ping 2012-01-02 499 ping 2012-01-03 495 ping 2012-01-04 493 ping 2012-01-05 496 ping 2012-01-06 498 pong 2012-01-01 - 5 pong 2012-01-02 - 3 pong 2012-01-03 - 5 pong 2012-01-04 -13 pong 2012-01-05 -10 pong 2012-01-06 - 6 How can I do this? there a are a lot of datasets (~1 million), so the query needs to be pretty cheap. I don't know how the use sum to get daily sums for daily records depending on the action-column.